Base stations in wireless communication systems provide wireless connectivity to users within the geographic area, or cell, associated with the base station. The wireless communication links between the base station and each of the users typically include one or more downlink (or forward) channels for transmitting information from the base station to the mobile unit and one or more uplink (or reverse) channels for transmitting information from the mobile unit to the base station. Multiple-input-multiple-output (MIMO) techniques may be employed when the base station and, optionally, the user terminals include multiple antennas. For example, a base station that includes multiple antennas can transmit multiple independent and distinct signals to multiple users concurrently and on the same frequency band. MIMO techniques are capable of increasing the spectral efficiency of the wireless communication system approximately in proportion to the number of antennas available at the base station.
It is well known that using a plurality of antennas instead of one antenna adds an additional spatial multiplex dimension to the communication system. One way to exploit the spatial dimension of multiple antennas in a MIMO system is to multiply complex weights to signals before transmitting the signals over each of the individual antennas of the multiple antennas at the base station. The complex weights are arranged in vectors, in which the vectors of complex weights form matrices. A set of matrices defined in a specific application scenario is called a codebook. An application scenario is related to a variety of parameters influencing the optimal codebook. Such parameters include antenna configuration parameters such as spacing and polarization and parameters characterizing the radio channel such as vehicular speed and scattering environment, for example. The operation performed on the signal before being transmitted from the base station is called precoding. Codebooks form an integral part of current mobile radio standards such as the first version (“Release 8”) of long term evolution (LTE) of the universal mobile telecommunication system (UMTS), as well as Worldwide Interoperability for Microwave Access (WiMAX) and Ultra Mobile Broadband (UMB), for example. The selection of an appropriate codebook is a function of the effective scenario and may vary over time. The effective scenario is a function of a large variety of parameters such as antenna configuration and radio environment and in a mobile radio environment that changes over time. As a consequence, the optimal codebook is also a function of time.
In order to deal with a large variety of scenarios (e.g., antenna configurations, and radio propagation channel conditions) encountered in real-world scenarios, a codebook may have to be changed or modified. Therefore, the possibility for mobile terminals to download a codebook used for precoding by base stations has been proposed for various applications in order to cope with all kinds of channel and antenna scenarios. The Ultra-Mobile Broadband standard developed in 3GPP2 includes downloadable codebooks. However, the signaling procedures by which such downloads are accomplished are cumbersome. For example, conventional methods of downloading codebooks must transmit a complex number for every element of every matrix in the codebook.